Direct oxidation of composite Al/Ti metal films as gate insulators for AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors (MOSHEMTs/HEMTs) is successfully realized.The devices fabricated with this novel process exhibit four orders of magnitude reduction in gate leakage current and remarkable breakdown voltage (Vbr =490 V vs 88 V for normal HEMT) improvement,compared with conventional Schottky-gate HEMTs.Furthermore,the transconductance of the MOSHEMT is only slightly lower (2.6％) than that of Schottky-gate HEMTs and have a wider full width of half maximum.The notable enhancement in device performance renders this new process highly promising for GaN-based microwave power amplifier applications in communication and radar systems.AlGaN/GaN high-electron mobility transistors (HEMTs) are expected to be applied in the highfrequency and high-power area because of their excellent properties.AlGaN/GaN has,for example,a wide energy band gap (3.39eV),high density (up to 1013 cm-2) two-dimensional electron gas (2DEG)and a high saturation electron velocity.Due to the unavailability of native oxide,Schottky-contact gates are commonly adopted in AlGaN/GaN HEMTs.